Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disorder characterized by selective loss of motor neurons which progresses rapidly to paralysis and death. Findings of decreased glutamate transport/uptake capacity and elevated extracellular glutamate levels in ALS patients suggest a role for glutamate excitotoxicity in the disease. Indeed, elevations in glutamate have been shown experimentally to induce selective Reactive Oxygen Species (ROS) production in motor neurons and a selective motor neuron injury, similar to ALS. A crucial missing link in understanding ALS is the factor responsible for this loss of glutamate transport (mediated primarily by glial cells). As glutamate uptake is ROS sensitive, we propose the following hypothesis: elevated extracellular glutamate selectively induces ROS production in motor neurons which directly inhibits glial glutamate uptake. Exploration of such a novel feed-forward model may elucidate new targets for potential therapeutic intervention. This proposal aims to examine the hypothesis using in vitro cell culture to characterize selective ROS production in motor neurons and whether it is capable of escaping the cell membrane and affecting adjacent glia, using oxidant sensitive fluorescent dyes and oxidative damage markers. Finally, studies in animal models of ALS will assess if these processes can be shown to occur in vivo.